A vehicle employing an internal combustion engine that uses gasoline and heavy oil as a main fuel causes too much pollution including air pollution. Recently, a lot of efforts have been put into developing an electric car or a hybrid electric vehicle (HEV) in order to reduce environmental pollution.
A high power secondary battery using non-aqueous liquid electrolyte of high energy consumption density has been recently developed. A plurality of high power secondary batteries are connected in serial to form a high-capacity secondary battery to be used in a device requiring high power for driving a motor such as the electric car.
As described above, one high-capacity secondary battery, which is hereinafter called “battery”, is generally formed of many batteries connected in serial. In case of the battery, particularly, the battery for HEV, it is required to be managed to maintain a proper operation state by controlling charging and discharging since several or tens of batteries alternatively perform charging and discharging
Accordingly, a Battery Management System (BMS) manages a general status of the batteries. The BMS estimates State of Charge (SOC) through an operation by detecting voltage, current, and temperature of the batteries, and controls the SOC of the battery such that a vehicle has the best fuel efficiency. The charging and discharging battery needs to be accurately measured to precisely control the SOC.
As a prior art, “a method for resetting soc of a secondary battery module” is disclosed in KR Patent Application No. 2005-0061123 (filed on Jul. 7, 20005).
The prior art provides a method for resetting SOC of a secondary battery module, comprising of: measuring a temperature value, a voltage value and a current value of battery modules during operation to accurately calculate SOC of a battery; calculating initial SOC based on the measured value; integrating currents; calculating actual SOC based on the integration value of currents; checking whether the battery module is in a non-load condition; when the battery module is in a non-load condition, checking whether the actual SOC is within a range of setting that it is measurably by integrating currents; and when the actual SOC is out of the range of setting, measuring a voltage value and calculating SOC based on the measured voltage value.
Generally, errors do not much occur in SOCi in the short term. However, as shown in FIG. 1, since the errors tend to be continuously integrated, a great deal of errors may occur in case that the battery operates for a long time. The integrated errors occur mainly when the battery is not completely charged or discharged. It is because errors due to skip of LBS digit in CPU for calculating SOC or reduction of charging amount by self-discharge largely affects accuracy. Also, the SOC accuracy greatly depends on a current measuring sensor. Accordingly, it is not possible to correct errors when there is a problem in the sensor.
On the other hand, as shown in FIG. 2, in case of SOCv, SOC is measured through electromotive force. This measuring method obtains an accurate result when current does not flow.
While current flows, accuracy in calculating of SOCv depends on charging and discharging patterns of the battery. Accordingly, the SOC may be less accurate according to the charging and discharging patterns. Since the charging and discharging patterns affecting the accuracy of SOCv are within a general usage range of a battery, it may cause many errors to use only the SOCv.